1. Field of the Invention
This invention relates to an improved lens mounting system which provides all the benefits of a simple sliding tube but without the associated drawbacks, and more particularly to a focusing mechanism which moves a focusing frame along the optical axis of each lens system linearly by constraining lateral, rotational and swing movements.
2. Description of the Prior Art
There are many ways to focus a lens. This invention concerns the focusing of a lens by means of mechanically changing its position along a fixed path, which maintains the orientation of the lens and follows the optical axis of the lens. This movement may be achieved by manual operation, or it can be motorized. This invention does not cover methods which involve using electronic or other means to change the shape or optical properties of a lens to achieve change in focus.
In an imaging device, when a lens is moved along its central optical axis, it is desirable for it not to shift or tilt with respect to this axis. This is because an imaging device requires an image capturing film or sensor to be placed in a fixed position at a predetermined focal plane. If the lens tilts or shifts, its optical axis and focal plane will tilt or shift with it. For conventional devices, it is not practical to move the image capturing sensor in concert with any tilt or shift of the lens. Any uncontrolled shifting or tilting will degrade the quality of the focus. Even a small tilt in the orientation of the lens will result in an image becoming blurred even when the lens has been moved to its correct point of focus on the optical axis. This means that the lens will effectively have to move along one optical axis which ends with the predetermined position of the image capturing sensor on the focal plane.
It is also desirable for the movement of the lens to be achieved with maximum efficiency to facilitate ease of use and economy of space. This is especially important where there is motorization or automation of the focusing mechanism.
Many systems have evolved to move a lens along its optical axis without shifting or tilting. This invention concerns systems which consist of a fixed constraining frame and a moving focusing frame on which the lens is mounted. The focusing frame, constrained by the constraining frame, guides the lens along its optical axis.
FIG. 1A depicts a concentric sliding tube lens focusing system consisting of a rigid outer focusing tube and a rigid inner focusing tube. An outer focusing tube 01 serves as a rigid housing to which a fixed constraining frame 02 is attached. An inner focusing tube serves as a moving focusing frame 03. A lens 04 is mounted in the focusing frame 03. The focusing frame 03 slides in and out of the constraining frame 02. Movement in all other axes other than the optical axis are constrained. The lens 04 cannot shift or tilt on its optical axis. It does however rotate. This movement of the lens relative to the focal plane 05 achieves the desired lens focusing function of the system. An image capturing device such as a CCD or film can be situated on the focal plane 05. A simple example of such a focusing device can be found in a basic pirate's telescope.
In practice, if there is full contact between the outer cylindrical surface of the focusing frame 03 and the inner cylindrical surface of the constraining frame 02, a large amount of friction will be introduced into the system.
For this reason, such systems are built with contact points at the two ends to support the inner focusing tube and locate the optical axis. The rest of the facing surfaces do not need to be in contact. This eliminates the possibility of the inner focusing tube being supported inadvertently at some point between the two end points, thereby shortening the length of the constraining frame. It also reduces friction and allows one tube to slide freely inside the other. The outer support tube functions as a constraining frame, because only the two contact points at either end are used for supporting the inner tube. The tube is merely a convenient shape for the device. It is unnecessary and difficult to manufacture rigid concentric tubes of sufficient precision to slide in and out of each other with full contact.
The key to a smoothly working concentric tube focusing system is the ratio of the diameter of the constraining joint as defined by the focusing frame to the length of the constraining frame. It is the two points of support at the extreme ends of the constraining frame which do the work of keeping the inner focusing frame in place while it is being moved.
FIG. 1B depicts a modified version of the focusing system in FIG. 1A with a constraining frame 10, focusing frame 11 and lens 04. It is good practice to design the tubes so that the ratio in length (A) of the constraining frame 10 to the diameter (B) of a constraining joint 07 is approximately 7 to 1 or greater. Otherwise, the focusing frame 11 carrying the lens 04 can rock within the constraining frame 10. The result would be that the lens 04 can deviate from its ideal position on the optical axis. 12.
In fact, depending on the size of the gap between the concentric tubes functioning as a focusing frame 11 within a constraining frame 10, if the ratio of A/B approaches 1, which would be a short constraining frame length for a given constraining joint diameter, the focusing frame 11 can tilt so much within its constraining frame 10 that it will jam and not slide at all.
In theory, one can design a lens tube with a small constraining joint, a long constraining frame and long focusing frame, and this will be a very stable and accurate focusing system. In practice, the focusing system in FIG. 1B needs to function as part of an overall imaging device. The constraining frame and focusing frame need to accommodate peripheral as well as principal light paths entering the imaging device, passing through the lens, and falling on the focal plane. The longer the frames, the narrower the angle of view.
The designer of the overall imaging device will desire as much room as possible on either side of the lens, and as much flexibility in the angle of view as possible. This can only be achieved by making the constraining frame and focusing frame as short as possible in order to reduce the thickness of the imaging device. This inevitably leads to the problems with a concentric tube system jamming when the ratio of the length (A) of the constraining frame 10 and the diameter (B) of the constraining joint 07 is insufficient.
Another implementation of the fixed constraining frame and moving focusing frame method is depicted in FIG. 1C. A constraining frame 14 with a helical thread and a focusing frame 13 with a matching helical thread are combined to form a strong but movable constraining joint. This constraining joint is strong, and prevents tilt and shift of the lens because it provides axial support with each thread surface. The ratio of constraining frame 14 to the constraining joint can therefore be reduced. Focusing systems using a helical method can have a short outer constraining frame 14 and large inner focusing frame 13 (large constraining diameter). Such a system prevents tilt and shift, but the lens 04 and focusing frame 13 will rotate within the constraining frame 14.
One variation of the helical thread system uses a double helical thread and two anti-rotation pins to move the lens plane in and out without the lens rotating at the same time.
In the system using concentric tubes and helical threads, the lens 04 and the focusing frame 13 are situated inside the constraining frame 14. This is a limitation inherent to the system.
Any helical thread, by nature, introduces drag and inefficiency. This is because the outer moving surface must move a long distance in order for the lens to move a short distance. Also, the total contact area is much greater than for a concentric tube system, where the focusing frame is only supported at two ends.
A focusing system involving helical threads is inherently more difficult and expensive to make than one using concentric tubes. It requires accurately cutting two mating and interchangeable helical threads. This is not easy to mass-produce. Mass-production in plastic of such high precision male/female helical thread tubes would require highly expensive precision molding equipment.
An auto focus lens with helical thread requires a high precision fit in which there is no slack and the fit is not too tight. This helps reduce the power required to drive the focusing mechanism and extends battery life. This balance is very difficult to attain in production.
FIG. 2A is a side view of a slider-and-track based focusing system. A fixed constraining frame 15 is in the form of a pair of tracks which support and constrain a focusing frame 16, which is attached to and guides a lens tube 17, in which is mounted lens 04. The lens tube 17 has a clearance fit in the rigid outer focusing tube 01. The constraining frame 15 is fixed to the outer focusing tube 01, but is no longer concentric with it or the lens tube 17.
An axis 18 of the focusing frame 16 is parallel to the optical axis 12. The tracks which act as the constraining frame may be cylindrical as shown in FIG. 2A, but may also be in some other shape. There are also track-based focusing systems which only use a single track. The focusing frame 16 can be moved along the constraining frame 15 using various methods, such as a rack and pinion system.
The position of the constraining frame 15 in FIGS. 2A, 2B serves to separate the constraining and focusing frame mechanism from the lens 04. The lens no longer needs to be situated in a pair of concentric tubes, which in turn would have their diameter determined by the size of the lens. This means that within the limits imposed by materials and workmanship, it is possible to design a sliding track with a very small diameter (A) of the constraining joint as defined by the focusing frame 16 and a length (B) of the constraining frame 15 similar to the length B of the constraining frame 08 in FIG. 1A.
In a track-mount based focusing system, there is no rotation of the lens. Tilt and shift are prevented by means of the constraining frame being made of very strong material, and the constraining frame and constraining joint being very precisely engineered to have no slack on the constraining joint while still allowing the smooth movement. This balance is very difficult to attain in production. It would also require the use of materials much stronger than plastic.